The present invention relates to an ignition timing control circuit for an internal combustion engine which is capable of controlling the ignition timing of the engine in a fine and precise manner, thereby ensuring the smooth operation thereof over substantially the entire operating range of the engine.
FIG. 5 shows a known ignition timing control circuit for an internal combustion engine. In this figure, a reference-signal pick-up coil 1 has one end coupled through a diode D.sub.1 to the base of a transistor Tr and the other end grounded, and generates a reference signal in the form of a pulse signal as a magneto generator (not shown) rotates. A resistor R.sub.1 has one end coupled to the cathode of the diode D.sub.1 and the other end grounded. A resistor R.sub.2 is coupled in series to the collector of a transistor Tr. A capacitor C.sub.1 is coupled at one end to the collector of the transistor Tr and grounded at the other end. The transistor Tr, the resistor R.sub.2 and the capacitor C.sub.1 together constitute a waveform shaping circuit. A flip-flop 2 including two NOR gates, a resistor and three capacitors has a set input terminal coupled to one end of the capacitor C.sub.1. The reference-signal pick-up coil 1, the waveform shaping circuit and the flip-flop 2 together constitute a reference signal generating means.
A counter 3 has a clock input terminal CLK coupled to a junction between one end of a resistor R.sub.8 and one end of a capacitor C.sub.2, a reset terminal R coupled to the flip-flop 2 and a plurality of output terminals Q.sub.1 .about.Q.sub.6. A digital to analog (D/A) converter 4 in the form of a rudder circuit including a resistance circuit network has a plurality (six in the illustrated example) of input terminals coupled to the corresponding output terminal Q.sub.1 .about.Q.sub.6 of the counter 3.
A load sensor 5 in the form of a throttle sensor comprising a resistor has one end coupled to the source of electricity and the other end grounded and generates an output voltage corresponding to the opening degree of a throttle valve (not shown). For example, the output voltage of the throttle sensor 5 varies in proportion to the opening degree of the throttle valve, as shown in FIG. 7. A function generator 6 is coupled to the throttle sensor 5 for generating a voltage as a function of the output voltage of the throttle sensor 5. The output voltage of the function generator 6 changes in relation to the degree of opening of the throttle valve in the manner as shown in FIG. 7. For example, the output voltage Vs of the function generator 6 takes a first constant value V.sub.1 when the opening degree of the throttle valve is in the range of from 0 to a first prescribed degree .theta..sub.1, then decreases as the opening degree of the throttle valve increases from the first prescribed degree .theta..sub.1 to a second prescribed degree .theta..sub.2, and finally takes a second constant value V.sub.2 when the opening degree of the throttle value further increases from the second prescribed degree .theta..sub.2, as clearly shown in FIG. 7.
A crank angle sensor in the form of a gear-signal pick-up coil 7 is disposed adjacent a ring gear operatively connected with the crankshaft of the engine for generating an output voltage corresponding to the rotational position of the ring gear, i.e., crank angle of the engine crankshaft. The gear-signal pick-up coil 7 has one end coupled to a resistor R.sub.3 and the other end grounded. A resistor R.sub.4 has one end coupled to the other end of the resistor R.sub.3 and the other end grounded. A resistor R.sub.5 has one end coupled to one end of a resistor R.sub.4 and the other end coupled to a junction between the anode of the diode D.sub.2 and the cathode of the diode D.sub.3. The cathode of the diode D.sub.2 is coupled to the source of electricity, and the anode of the diode D.sub.3 is grounded. The resistor R.sub.6 has one end coupled to the other end of the resistor R.sub.5 and the other end coupled to the non-inverted input terminal of a comparator 8 which will be described later in detail. A resistor R.sub.7 has one end coupled to the other end of the resistor R.sub.6 and the other end grounded. The comparator 8 has an inverted input terminal grounded. An inverted amplifier 9 is coupled to the output terminal of the comparator 8. A multiplier 10 in the form of a twofold multiplier 10 including an inverted amplifier, resistors, capacitors and a NAND gate is coupled to the output terminal of the inverted amplifier 9. An inverted amplifier 11 has an input terminal coupled to the twofold multiplier 10 and an output terminal coupled to one end of a resistor R.sub.8 which is coupled at the other end to one end of the capacitor C.sub.2 which is grounded at the other end. The junction between the resistor R.sub.8 and the capacitor C.sub.2 is coupled to the clock input terminal CLK of the counter 3. Thus, the gear-signal pick-up coil 7, resistors R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7 and R.sub.8, diodes D.sub.2 and D.sub.3, comparator 8, inverted amplifier 9, multiplier 10, inverted amplifier 11 and capacitor C.sub.2 together constitutes a clock pulse generator.
A comparator 12 has a non-inverted or positive input terminal coupled to the output side of the D/A converter 4 and an inverted or negative input terminal coupled to the output side of the function generator 6. A one-shot multivibrator 13 has an input side coupled to the output terminal of the comparator 12 and an output side coupled to the reset terminal of the flip-flop 2 and to an amplifier 14 which is connected with an ignition device, i.e., a primary side of an ignition coil.
FIGS. 6(a) through 6(g) illustrate the operational waveform of the various portions of the known ignition timing control circuit of FIG. 5, which will be described later in detail.
With the known ignition timing control circuit as constructed above, the reference-singal pick-up coil 1 generates a pulse signal containing positive and negative pulses (only two pulses are illustrated here for the sake of simplification) at predetermined rotational positions of the rotary shaft of the magneto generator (not shown), as shown in FIG. 6(a), as the magneto generator rotates. The positive pulses of the pulse signal are imposed through the diode D.sub.1 on the base of the transistor Tr so that they are waveform shaped by the transistor Tr, the resistor R.sub.2 and the capacitor C.sub.1 to provide corresponding rectangular-shaped pulses (only one is shown in FIG. 6(b)), which are input to the set input terminal of the flip-flop 2, thus switching it. As a result, the output at the inverted output terminal of the flip-flop 2 goes to the low level, as shown in FIG 6(c), so that the counter 3 starts counting.
On the other hand, the gear-signal pick-up coil 7 generates a wave signal having a saw-tooth waveform, as shown in FIG. 6(d), as the ring gear (not shown) operatively connected with the crankshaft (not shown) rotates. The saw-tooth wave signal thus generated contains triangular-shaped pulses each corresponding to the respective one of the teeth internally formed on the ring gear. The saw-tooth signal of the gear-signal pick-up coil 7 is imposed on the comparator 8 where it is waveform shaped and then supplied through the inverted amplifier 9 to the twofold multiplier 10 which provides rectangular-shaped clock pulses having a frequency two times greater than that of the saw-tooth wave signal, as shown in FIG. 6(e). The clock pulses thus produced are input to the clock input terminal CLK of the counter 3 so that the counter 3 counts the number of the clock pulses thus input and produces as digitalized signal in the form of a binary number repesentative of the counted clock pulse number at the outpout terminals Q.sub.1 .about.Q.sub.6 each time when the number of the input clock pulses counted reaches six distinct predetermined levels. For example, the output terminal Q.sub.1 .about.Q.sub.6 correspond respectively to six different but successive counting levels, the terminal Q.sub.1 corresponding to the lowest one and the terminal Q.sub.6 to the highest one. The digitilized output thus produced is fed to the D/A converter 4 where it is converted into an analog voltage of stepped form, as shown in FIG. 6(f), which is then imposed on the non-inverted input terminal of the comparator 12. On the other hand, imposed on the inverted input terminal of the comparator 12 is the output voltage Vs of the function generator 6 to which the output voltage of the throttle sensor 5 is input.
Accordingly, the stepped voltage supplied to the comparator 12 from the D/A converter 4 is compared with the output voltage Vs from the function generator 6 so that when the stepped voltage increases to a level higher than the function generator output voltage Vs, the output of the comparator 12 changes from the low to the high level, as shown in FIG. 6(g). The high-level output of the comparator 12 triggers the one-shot multivibrator 13 to produce a waveform-shaped pulse signal which is in turn amplified by the amplifier 14 to provide an ignition timing control signal which is fed to the unillustrated ignition coil. At the same time, the output signal of the one-shot multivibrator 13 is also input to the reset input terminal on the flip-flop 2 so that the flip-flop 2 is reset to return the output thereof to the high level. As a result, the high level output of the flip-flop 2 is input to the reset terminal R of the counter 3 whereby the counter 3 is reset to stop counting and placed in a condition ready for the next counting.
With the above-described known ignition timing control circuit of the gear counting type utilizing a ring gear, in which ignition timing is controlled by detecting the crank angle of the engine, however, the output signal of the gear signal pick-up coil 7 containes a saw-tooth wave having a relatively large angular wave width which corresponds to the shape or angular width of the respective internal teeth of the ring gear. Therefore, it is extremely difficult to obtain a wave signal having a continuously changing waveform due to restrictions on the shape and arrangement of the ring gear teeth. Thus, ignition timing can only be controlled stepwise with a relatively large angular width or distance as the output voltage Vs of the function generator 6 continuously changes. This makes it difficult for the engine to operate in a smooth manner over a wide range of engine operation from a low to high speed. Such a tendency is particularly evident when the rotational speed or load of the engine is low.